Early diagnosed and treated infants with PKU will have normal growth and development. Women with PKU who are treated prior to and during pregnancy will have pregnancy outcomes comparable to non-PKU women. Children with PKU and childbearing women are monitored nutritionally, biochemically, physically and developmentally to test these hypotheses and to establish a standard of care for this disorder. Children with PKU who are not early treated and carefully monitored become mentally retarded. Women with PKU who are not treated and monitored before and during pregnancy have delivered offspring with severe irreversible birth defects including mental retardation. An application for an extension of the Maternal PKU for four additional years is in progress. The purpose of the extension is to follow the growth and development of the study offspring as they become school age to more fully answer the research question i.e. Are children of PKU women comparable to children of non-PKU women if the PKU woman is on diet and in good metabolic control. An additional phase of the study is the DNA analysis of the women, offspring and grandmothers. The research questions are: Is the genotype of the woman with PKU predictive of birth outcome? Will identifying the mutation assist clinicians in prescribing the low phenylalanine diet?